Treatment of steel surfaces



steel reaction tubes for reuse.

Patented June 23, 1942 TREATMENT OF STEEL SURFACES John P. Loud, Cumberland, Md., assignor to Celanese Corporation of America, a corporation of Delaware No Drawing.

Application August 31, 1940, Serial No. 355,041

' Claims. (Cl. 148-8) This invention relates to the treatment of alloyed steel surfaces, and relates more particularly to the treatment of alloyed steel surfaces to remove decomposition products therefrom.

An object of my invention is to clean alloyed A further object of my invention is to remove carbonaceous deposits from alloyed steel reaction tubes.

Another object of my invention is the provision of a novel process .for treating alloyed steel reaction tubes to remove carbonaceous decomposition products therefrom whereby any con- 7 tinned and accelerated decomposition is inhibited.

Still another object of myinvention is to remove said decomposition products rapidly and economically without any deleterious effects upon the continued usefulness of the tubes.

Other objects of my invention will appear from the following detailed description.

In carrying out organic chemical reactions upon an industrial scale, wide use is made of various alloys of iron. Frequently the reactants being treated are,subjected to'elevated temperatures and pressures with the result that a certain amount of decomposition and carbonization occurs. By careful control such decomposition and carbonization of the reactants may be decreased, but over long periods of time the cumulative effect builds up a layer of carbonized material which reduces the eiliciency of the process. In the case of alloyed steel tubing, not only is the rate of heat transfer of the tubes cut down but their effective diameter is also diminished due to the presence of the carbonized layer on the inner surfaces. When the effect becomes excessive, the process must be interrupted and the reactors and tubing must be cleaned and refinished. Heretofore, straight tubes have been rebored while larger vessels were cleaned by chipping the carbonaceous deposits from the affected surfaces. In the case of coiled tubing, boring was not always feasible and hammering these scaly deposits loose injured the tubing. Thus, in some instances the only solution to the problem was to replace the affected par-tswithentirely new ones. Such replacement is extremely costly, particularly in specially fabricated chemical processing equipment, and constitutes a serious economic loss. s

I have now discovered that such carbonaceous organic decomposition products may be removed from alloyed steel surfaces by chemical means without the necessity for drastic physical'treatment. According to my invention, alloyed .steel surfaces upon which there is a coating of car-'' bonaceous organic decomposition products may be refinished and saiddecomposition products removed by washing the surface with'wet steam, soaking the surface in a heated dilute solution of inorganic acids, comprising a mixture of an oxidizing inorganic acid and a halogen acid,

washing the surface with water, soaking it again in a heated and more concentrated solution of an oxidizing inorganic acid and then washing the surface again to free it of all acids. By this treatment, the carbonized decomposition products deposited upon the steel surface are removed and the surface is restored substantially to condition. The heat transfer rate through the steel is brought up to normal-"and the original effective diameter of the treated tuboriginal ing is restored.

The length of time the carbonized surfaces be subjected to the action of steam will vary depending upon the amount and density of deposited material, which must be acted upon and prepared for the chemical treatment which follows. Generally, subjecting the steel surface to the action of the steam for-from about 6 to 24 hours 'is sufllcient. Steam containing from about 5% to 20% moisture may be used.' Steam at atmospheric pressure may be used or else steam up to pressuresv of about 150 lbs. per square inch may be used if a higher temperature is desired.

The inorganic oxidizing acids which may be used for cleaning steel surfaces according to my invention are acids such as nitric acid, chromic acid and the like. In the initial acid treatment the acid may be used in concentrations of from about 6% to about 12% and containing from about /2% to 3% of a halogen acid; Such halogen acids as hydrofluoric acid maybe used. In the initial acid treatment the temperature of the solution may be maintained at from about C. to about C. and the solution maybe permitted to react on the steel surface for from about 2 to 6 hours. In the second acid treatment, where a more concentrated solution of the oxidizing acid is used, the concentration of the acid may be from-about 10% to about 20%. The

steel surface is subjected to the action of the acid solution for a shorter period of time in the second acid treatment and this period maybe from about 1 to 2 hours. The temperature of the acid solution during the second acid treat-- ment may be maintained at from about 70 C. to about 90 C. V

In order further to illustrate his invention,

but without being limited thereto,

Example Alloyed steel tubing which has been used for the preheating of glacial acetic acid and whose inner surface has been heavily carbonized and impregnated with carbon after long usage, is subjected to the action of wet steam at a pressure of 50 lbs. per square inch and containing of moisture for 24 hours. The tubing is then soaked in an aqueous solution of 10% nitric acid and 1% hydrofluoric acid at 80 C. to 90 C. for six hours. The tubing is then washed well with water and soaked in a 20% aqueous solution of nitric acid at 80 C. to 90 C. for two hours. The tubing-is again washed well with water to remove all traces of the acid. By this treatment the heavily carburized inner surface is thoroughly cleaned and substantially restored to its original uncoated condition. With the carbonized products removed the corrosion-resistance of the tubes is enhanced and the continued decomposition of the acid inhibited.

It is to be understood that the foregoing detailed description is merely given by way of illustration and that many variations may be made therein without departing from the spirit of. my invention.

Having described my invention, what I desire to secure by Letters Patent is:

1. A process for treating carburized alloyed steel surfaces, which comprises subjecting 'said steel surfaces to-the action of steam, treating the surfaces with a relatively dilute aqueous solu tion of an oxidizing inorganic acid selected from the group consisting of nitric acid and chromic acid and a hydrohalide acid, and treating said surfaces again with a more concentrated aqueous solution of such oxidizing inorganic acid.

2. A process for treatingcarburized alloyed steel surfaces, which comprises subjecting said steel surfaces to the action of steam, treating the 5. A process for treating carburized steel surfaces, which comprises subjecting said steel surfaces to the action of steam, treating the surface with about a 10% aqueous solution of nitric acid containing about 1% of hydrofluoric acid, said solution being maintained at about 80 C. to 90 C., and treating said surfaces again with an aqueous solution containing about 20% of nitric acid and maintained at about 80 C. to 90 C.

6. A process for treating carburized steel surfaces, which comprises subjecting said steel surfaces to the action of wet steam, treating the surfaces with a relatively dilute aqueous solution of an oxidizing inorganic acid selected from the group consisting of nitric acid and chromic acid and a hydrohalide acid, washing said surfaces with water, treating said surfaces again with a more concentrated aqueous solution, of such oxidizing inorganic acid and then washing them free of acid with water.

7. A process for treating carburized steel surfaces, which comprises subjecting said steel surthe surfaces with a heated aqueous solution containing from about 6% to about 12% of an oxidizing inorganic acid selected from the group consisting of nitric acid and chromic acid and from about to about 3% of a hydrohalide acid, washing said surfaces with water, treating said surfaces again with a heated aqueous solution containing from about 10% to about. 20% of such oxidizing inorganic acid, and then washing them free of acid with water.

9. A process for treating carburized alloyed steel surfaces, which comprises subjecting said steel surfaces to the action of wet steam, treating heated aqueous solution of such oxidizing inorganic acid.

3. A process for treating carburized alloyed steel surfaces, which comprises subjecting said steel surfaces to theaction of steam, treating the surfaces with a heated aqueous solution containing from about 6% to about 12% of an oxidizing inorganic acid selected from the group consisting 'of nitric acid and chromic acid and from about /2% to about 3% of a hydrohalide acid, and

treating said surfaces again with. a, heated aqueous solution containing from about 10% to about 20% of such oxidizing inorganic acid.

4. Process for treating carburized alloyed steel surfaces, which comprises subjecting said steel about 3% of hydrofluoric acid, andtreating said surfaces again for from about 1 to 2 hours with a heated aqueous solution containing from about 10% to about 20% of nitric acid.

the surfaces for from 2 to 5 hours, with a heated aqueous solution containing from about 6% to about 12% of nitric acid andfrom about to about 3% of hydrofluoric acid, washing said surfaces with water, treating said surfaces again for from 1 to 2 hours with a heated aqueous solution containing from about 10% to about 20% of nitric acid, and-then washing them free of acid with water.

10. A process for treating carburized alloyed steel surfaces, which comprises subjecting said steel surfaces to the action of wet steam, treating the surfaces with about a 10% aqueous solution of nitric acid containing about 1% of hydrofluoric acid, said solution being maintained at about C. to C., washing said surfaces with water, treating said surfaces again with an 4 aqueous solution containing about 20% of nitric acid and maintained at about 80 C. to 90 C., and then washing them free of acid with water.

JOHN P. LOUD. 

